Research Article |
Corresponding author: James K. Wetterer ( wetterer@fau.edu ) Academic editor: Petr Klimeš
© 2017 James K. Wetterer.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Wetterer JK (2017) Invasive ants of Bermuda revisited. Journal of Hymenoptera Research 54: 33-41. https://doi.org/10.3897/jhr.54.11444
|
For 60+ years, two of the world’s most widespread and destructive invasive ant species, the African big-headed ant (Pheidole megacephala) from tropical Africa and the Argentine ant (Linepithema humile) from subtropical South America, have been engaged in an epic battle on the islands of Bermuda. Both species are completely intolerant of the other and are also well-known for killing off native invertebrates, particularly other ants. Here I surveyed sites across Bermuda in 2016, including resurveys of the locations previously surveyed in 1963, 1966, 1973, 1986, and 2002, to provide an update on this conflict. The status of all other ant species present in the islands, including previous records from literature, is also provided. In addition, I surveyed ants nesting in red mangrove (Rhizophora mangle) trees to evaluate whether this arboreal habitat may serve as a refuge for previously overlooked ant species.
In 2016, L. humile occurred at most surveyed sites in Bermuda, including all ten resurveyed sites. Pheidole megacephala was present at only two resurveyed sites, a lower proportion of sites than any of the five earlier surveys. Still, P. megacephala occupied substantial areas, particularly in and around Hamilton, the capital of Bermuda. This survey increased the number of ant species with verified records from Bermuda to 25, including four exotic species recorded for the first time: Cardiocondyla minutior, Pheidole navigans, Strumigenys emmae, and Strumigenys membranifera. I found five ant species nesting in mangroves: L. humile plus four Old World exotics, C. minutior, Cardiocondyla obscurior, Monomorium floricola, and Plagiolepis alluaudi.
It appears that L. humile may be better suited to the subtropical climate of Bermuda than P. megacephala, except perhaps in warmer and sunnier habitats, such as plantings along urban streets and in open parks, where P. megacephala may hold the advantage. The dataset on the new ant records from 2016 is provided with the paper.
Atlantic islands, biodiversity, exotic ants, Pheidole megacephala , Linepithema humile , tramp ants
For more than 60 years, two of the world’s most widespread and destructive invasive ant species, the African big-headed ant (Pheidole megacephala) from tropical Africa and the Argentine ant (Linepithema humile) from subtropical South America, have been engaged in an epic battle on the Atlantic islands of Bermuda. Pheidole megacephala was first recorded in Bermuda in 1889 (Dahl 1892), and was the dominant ant species there in the early 20th century (
Both P. megacephala and L. humile are well-known for killing off native invertebrates, particularly ants (
In the present study, I surveyed sites across Bermuda to provide an update on this conflict and on the status of other ant species present. In addition, I surveyed ants nesting in red mangrove (Rhizophora mangle) to evaluate whether this arboreal habitat may serve as a refuge for some previously overlooked ant species. Red mangroves grow in and adjacent to shallow brackish water in subtropical and tropical estuaries around the world, providing a unique habitat, often completely isolated from any terrestrial habitat.
In 3–10 May 2016, I surveyed ants in Bermuda. I hand-collected ants using an aspirator, sifted litter using a Davis sifter, and collected ants from inside dead twigs and branches of red mangrove.
To examine changes in which ant species, P. megacephala or L. humile, dominated specific areas in Bermuda,
1. Great Head Park (32.368, -64.650; 9 May 2016)
2. Mullet Bay Road and Ferry Road intersection (32.375, -64.696; 5 May 2016)
3. Leamington Caves (32.342, -64.708; 4 May 2016)
4. Knapton Hill Road and Harrington Sound Road intersection (32.321, -64.716; 4 May 2016)
5. Knapton Hill Road and Harrington 100s Road intersection (32.315, -64.724; 5 May 2016)
6. Christchurch Lane and Brighton Hill Road intersection (32.306, -64.754; 9 May 2016)
7. Spittal Pond Reserve (32.311, -64.724; 5 May 2016)
8. Newstead Hotel (32.284, -64.787; 9 May 2016)
9. Wreck Road (32.276, -64.881; 8 May 2016)
10. Ireland Island (32.327, -64.835; 8 May 2016)
I collected dead twigs and branches of red mangrove into 3.8-liter zip-lock storage bags (3–10 bags from different mangrove trees, depending on the number of accessible trees) at 14 sites in Bermuda (geo-coordinates, collection date, and # of bags in parentheses):
1 Tom Wood’s Bay (32.304, -64.814; 3 May 2016; 10)
2. Hungary Bay (32.291, -64.760; 3 May 2016; 10)
3. Crow Lane Park (32.293, -64.771; 3 May 2016; 10)
4. Mangrove Lake (32.326, -64.711; 4 May 2016; 10)
5. Trott’s Pond (32.330, -64.702; 4 May 2016; 10)
6. Walsingham Bay (32.346, -64.709; 4 May 2016; 10)
7. Coot Pond (32.389, -64.678; 5 May 2016; 4)
8. Mullet Bay Rd and Ferry Rd (32.375, -64.696; 5 May 2016; 4)
9. Stokes Point Park (32.371, -64.694; 5 May 2016; 3)
10. Blue Hole (32.351, -64.710; 5 May 2016; 10)
11. Fairyland Creek (32.295, -64.802; 6 May 2016; 10)
12. Mill Creek (32.304, -64.801; 7 May 2016; 10)
13. The Lagoon (32.313, -64.844; 8 May 2016; 10)
14. Pilchard Bay (32.276, -64.880; 8 May 2016; 10)
I also did hand collecting and litter sifting in areas adjacent to each red mangrove sites, as well as at numerous other sites when time and weather permitted. On the last day of collecting (10 May 2016), I surveyed 12 urban sites in and around Hamilton city, the capital of Bermuda. In addition, three people (R. Franco, J. Steele, and C. Stringer) sent me samples of ants they collected in and around their homes in Bermuda in May 2016.
I retained all pinned and alcohol specimens in my personal collection.
In 2016, I collected a total of 19 ant species in Bermuda, including four recorded for the first time: Cardiocondyla minutior, Pheidole navigans, Strumigenys emmae, and Strumigenys membranifera (Table
In their 2002 survey,
At two long-term survey sites, I found both L. humile and P. megacephala: on Ireland Island and the Newstead Hotel (now Newstead Belmont Hills Golf Resort and Spa). On Ireland Island, I found P. megacephala along the North Breakwater and by the Maritime Museum (now the National Museum) as before. In addition, I collected P. megacephala in front of the Clocktower Mall and to the south end of the Glassworks Mall, two places occupied by L. humile 14 years earlier, indicating a modest expansion of the P. megacephala population on North Ireland Island. At the Newstead Hotel, I found the boundary between L. humile and P. megacephala territory, near the western edge of the property, essentially identical as 14 years earlier. At the Newstead Hotel, I collected in the same vial L. humile and P. megacephala workers from only a few meters apart; the ants immediately locked in battle, confirming their mutual intolerance.
I found five ant species nesting in red mangrove in Bermuda: L. humile plus four Old World exotics that
At many red mangrove sites, I saw streams of L. humile on the branches of red mangrove growing in shallow water. In some cases, L. humile workers I found inside twigs could have belonged to land-based colonies, where they could access the mangrove growing in water via a continuous canopy. However, one sample from Pilchard Bay that had L. humile inside twigs came from a small mangrove island consisting of two mangrove trees isolated by water from all other mangroves. This L. humile colony had to be completely arboreal.
All three samples that people sent me from their houses included L. humile. One also included P. megacephala and Brachymyrmex cf. heeri, and one included P. navigans.
Ants of Bermuda, arranged in descending order of the number of collection sites in 2002 (
Species | 2002 | 2016 | Dates | Status |
---|---|---|---|---|
Linepithema humile | 27 | 38 | 1948–2016 | NX |
Pheidole megacephala | 17 | 17 | 1889–2016 | OX |
Brachymyrmex cf. obscurior | 19 | 9 | 1905–2016 | native? |
Brachymyrmex cf. heeri | 6 | 8 | 1905–2016 | native? |
Solenopsis sp. thief ant | 4 | 8 | 1934–2016 | native? |
Paratrechina longicornis | 7 | 4 | 1990–2016 | OX |
Hypoponera opaciceps | 4 | 4 | 1905–2016 | native? |
+Pheidole navigans | 6 | 2016 | NX | |
Tetramorium simillimum | 2 | 4 | 1922–2016 | OX |
Tetramorium caldarium | 1 | 4 | 2002–2016 | OX |
Monomorium floricola | 4 | 2009–2016 | OX | |
Odontomachus ruginodis | 1 | 2 | 1889–2016 | native? |
+Strumigenys membranifera | 3 | 2016 | OX | |
Camponotus pennsylvanicus | 2 | 2001–2002 | NX | |
Cardiocondyla emeryi | 1 | 1 | 1905–2002 | OX |
Cardiocondyla obscurior | 2 | 1987–2016 | OX | |
Monomorium monomorium | 1 | 1 | 1900–2016 | OX |
Plagiolepis alluaudi | 2 | 1945–2016 | OX | |
+Cardiocondyla minutior | 1 | 2016 | OX | |
+Strumigenys emmae | 1 | 2016 | OX | |
Crematogaster sp. male | 1987 | native? | ||
Wasmannia auropunctata | 1925–1966 | NX | ||
Nylanderia vividula | 1905–1925 | OX | ||
Hypoponera punctatissima | 1910 | OX | ||
Nylanderia cf. fulva | 1905 | NX |
Ten sites surveyed repeatedly by
Site | Year | |||||
---|---|---|---|---|---|---|
1963 | 1966 | 1973 | 1986 | 2002 | 2016 | |
Great Head Park | – | – | both | L | L | L |
Mullet Bay Rd. & Ferry Road | P | both | both | L | L | L |
Leamington Caves | L | – | P | L | L | L |
Knapton Hill Rd Intersection | L | – | both | L | L | L |
Knapton Hill Rd & Harrington 100s | L | – | P | L | both | L |
Christchurch & Brighton Hill | both | – | P | L | L | L |
Spittal Pond | P | – | P | P | both | L |
Newstead Hotel | L | – | L | both | both | both |
Wreck Road | both | – | P | P | L | L |
Ireland Island | – | P | – | P | both | both |
% observations = P. megacephala | 40 | 67 | 67 | 36 | 29 | 17 |
In 2016, I found that L. humile dominated most sites I surveyed in Bermuda. Still, P. megacephala occupied substantial areas, particularly in and around the capital of Hamilton. Pheidole megacephala is a dominant species in tropical sites around the world, whereas Linepithema humile tends to dominate in subtropical areas with “Mediterranean” climates. It appears that L. humile may be better suited to the subtropical climate of Bermuda, except perhaps in warmer and sunnier habitats, such as the plantings along urban streets and in open parks, where P. megacephala may have the advantage.
One ant species that is possibly native to Bermuda is the trap-jaw ant Odontomachus ruginodis. This species was once common in Bermuda, but now appears to be quite rare (
Bermuda has the northernmost populations of mangroves in the world (
In the absence of invader-free refuges, invasive ants can drive native species to extinction. For example, in 2003, I surveyed ants on Monte Gordo, the highest mountain of the Atlantic island of São Nicolau, Cape Verde, and found only P. megacephala at all sites sampled except for sites within 100 m of the mountain’s peak (JKW, unpublished data). Here, where P. megacephala had not yet spread, a small pocket of other ant species survived, including an endemic species known only from this mountain, Monomorium boltoni. If P. megacephala continues its spread up the mountain, it seems unlikely that M. boltoni will survive once P. megacephala overruns what may be its last refuge. Similarly, it may be that all ant species originally native to Bermuda are now extinct.
I was surprised to collect P. navigans at five sites scattered across Bermuda (and I was sent specimens from a sixth site), given that this species had never before been collected in Bermuda, and that I surveyed three of these same locales in 2002. This suggests that P. navigans may be a recent arrival on Bermuda and is spreading quickly. Curiously, at four of the five sites, P. navigans was coexisting with L. humile. On Ordnance Island, I even found them nesting together under the same piece of concrete. It would be interesting to determine whether or not P. megacephala can tolerate P. navigans. Pheidole navigans, a big-headed ant native to the Neotropics, is known as an exotic in the southeastern US (formerly misidentified as P. moerens; see
Pheidole megacephala and L. humile began their worldwide dispersal in the 19th century and have already spread through much of their potential range (
I thank M. Wetterer for comments on this manuscript; L. Greene, C. Jessey, and S.R. Smith for help and hospitality in Bermuda; R. Franco, J. Steele, and C. Stringer for sending me samples from their homes; Florida Atlantic University for financial support.
Specimen collection information
Data type: specimens data
Explanation note: Self-explanatory